Welcome to Healthcare Magic forum.
Though intercourse just the day before periods or in last half of menstrual period has very less chance of pregnancy
since the ovum has already degenerated, one cannot say it is 100% protective.Also the sperm can remain viable in the female genital tract for 3_5 days so a conception in the next menstrual cycle is a possibility.
So in your case it would be best to confirm it using urine pregnancy test kit preferably using a first void morning sample.I assume you had regular cycles before this because the other possibilities if pregnancy is ruled out are, hormonal
, uterine disorders (fibroids), or ovarian pathology (pcod
In any case it is not worth waiting more and an evaluation will clear all doubts.
Hope I have answered your query.